This study is an exploration of the human brain processing of pain using functional MRI on humans. The goal is to develop objective quantitative assay of pain. The subjects will be patients with a specific type of pain, postherpetic neuralgia, an aftermath of herpes zoster. Such patients have pain is localized to the nerve affected by the herpes zoster and is always unilateral. The severity of the pain increases greatly when the localized area of the nerve is touched with external stimuli such as brushing. We will examine brain activation pattern in response to painful stimulation. Stimulation on contralateral non-painful site will be used as control. Normal subjects will also serve as controls. A stimulating apparatus for use in magnet bore has been constructed, which allows a mechanical stimulus on patient body surface area with a controlled amount of force. We are starting to collect data with the exerted force varied, and oberve the change in intensity in the brain images.